DEAR CAR TALK: Our 2018 Mercedes C300 lease is expiring soon. The lease states that the residual value is $22,000. But when I looked it up online, I found out the car is now worth $30,000.

Should we buy it "off lease" and sell it ourselves? Could Mercedes want to charge us more for the car than the residual listed in the lease? — Gene

DEAR READER: Yes, you should buy the car at the end of the lease and sell it yourself.

A lease is a contract, and Mercedes can't change it now that the car is suddenly worth more than they expected.

If the car was worth less than $22,000 now, do you think Mercedes would have said, "OK, just give us $12K, Gene, and we'll call it even?"

Here's how a lease works. When you lease a car, the car company estimates what the vehicle will be worth at the end of the lease period.

Let's say the car sells for $30,000. The company, based on lots of historical data, estimates that after 36 months and 36,000 miles, the car will be worth, say, $20,000. What you pay during the lease is the difference between those two numbers. So, in this case, you'll pay $10,000 over the 36 months you use the car, plus some interest.

At the end of the lease period, you have the right to either buy the car for that predetermined residual value, or you can turn it in and walk away.

But here's what's different at the moment: Due to new vehicle shortages and the resulting big increases in used car prices, many lessees are now finding that their cars are worth more — often much more — than the estimated residual value. Hey, stuff happens!

So you have two choices, Gene. You can either buy the car at a discount and make a profit on it yourself, or you can let the dealer keep it and make a profit on it. That's an easy one, right?

Dealerships fully understand what's going on here and will be trying hard to convince you to just turn in your leased car. They'll offer you small perks (i.e., one free lease payment) and hope you haven't realized that you can make thousands by buying it yourself. Or save thousands by buying it at a discount and keeping it.

One caveat, Gene: Before you decide to buy and resell the car, check with your state Department of Motor Vehicles regarding sales tax. Normally, when you buy a car, you have to pay sales tax on the purchase. But many states give you a 10-day grace period — so if you buy the car off-lease and sell it within 10 days, you'll avoid the tax. So, time your move carefully and enjoy the windfall.

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DEAR CAR TALK: My friend, Greg, and I coincidentally bought the same make, model and color of car and in the same year! It's a 2WD 2010 Lexus RX-350.

We are both careful to follow the recommended maintenance specified in the owner's manual. For normal driving, the manual does not list changing the differential fluid as routine maintenance. It's recommended only if towing or driving in excessively dirty or dusty conditions. There is no dipstick, and changing the fluid is difficult.

Greg recently changed his, and the fluid was dark like prune juice (yum). Would you recommend it be changed, and, if so, how often? — Jeff

DEAR READER: Well, I'll give you two answers, Jeff.

From the point of view of a mechanic, it certainly can't hurt to the change the differential fluid.

This car uses a synthetic, long-life differential fluid. But the owner's manual indicates that under heavy-duty use (like towing, high temperatures), when the fluid gets hotter, changing the fluid is beneficial. After all, excessive heat is what breaks down every lubricant.

So you might not need to change it, but it's clear that no harm will come from replacing it. And possibly some benefit — especially on an older, high-mileage car.

And by the way, if your pal Greg checked his brand-new differential fluid after driving for two weeks, it would also look like prune juice. That's just how used differential fluid looks — it's disgusting — even after minimal use.

My second answer for you is from the point of view of a scientist, Jeff. You have set the stage for a perfect experiment. Same car, same age, same maintenance program — except for the differential fluid change.

The scientist in me would tell you — by all means — don't change your fluid. Instead, take the $150 and treat your spouse to a nice dinner out. And then keep driving and see what happens.

Let me know which runs longer, your differential or his marriage.

Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting cartalk.com